1. Field of the Invention
The present invention is directed to a method as well as an amplifier circuit for distortion suppression of regulated signal amplification in a hearing aid.
2. Description of the Prior Art
Currently, hearing aids usually have an amplifier with gain control (AGC) that is dependent on the signal level. This gain control makes it possible to amplify the signals in such a way that they are clearly audible to the user while still maintaining the sound sensation pleasant for the user given high input signals.
Given an amplifier with gain control that is dependent on signal level, harmonic distortion that the user considers disturbing occurs due to the control function. This can be clearly seen given a constant sine input signal, whereby a constant re-adjustment is required even in the steady state. The re-adjustment intensity is dependent on the decay time of the amplifier circuit. The harmonic distortion is directly dependent on the re-adjustment intensity, and thus is indirectly dependent on the decay time. Longer decay times are required in order to achieve low harmonic distortion. Given the minimal decay times that are currently standard and desirable in hearing aids, the harmonic distortion can only be kept within a tolerable degree with considerable technical outlay. As a result, standards for hearing aids are partly difficult to meet or cannot be met at all in certain cases.
German OS 23 53 696 discloses a hearing aid with an automatic gain control for which a number of different response/decay times are prescribed between which changes optionally can be made. The setting ensues via adjustment means that are manually actuated.
Further, German OS 19 27 848 discloses a hearing aid that has a multi-stage transistor amplifier with an input to which a microphone is connected via two signal lines, and a stage in the proximity of the amplifier output containing a gain control circuit that acts automatically from its output onto its input, and the input of the stage contains a voltage divider in the first signal line. This voltage divider is composed of a fixed resistor and a resistor branch composed of two diodes connected with opposite polarity in terms of alternating current that is variable dependent on the output signal. As a result, the delays appearing in corresponding circuits can be avoided since the d.c. circuit of the amplifier itself remains uninfluenced.